The Federal Communication Commission (FCC) originally allocated frequencies of 870-890 megahertz (mhz) for transmission and 825-845 mhz for reception of cellular communications. The channel bandwidth was chosen at 30 kilohertz (khz) with transmission, reception separation at 45 mhz. During this initial allocation of frequencies, the FCC further sub-divided the receive and transmit bands into ten megahertz sub-bands designated as non-wireline and wireline sub-bands. The non-wireline service is typically provided by any private entrepreneur who has obtained licensing rights through the FCC and other governmental agencies. The wireline service is provided by the regional telephone company where the cellular communications are resident. In any region where cellular service is to be provided, it can be served by one non-wireline service and one wireline service.
The sub-bands for reception were divided into 825-835 mhz for non-wireline service and 835-845 mhz for wireline service. Similarly, the transmit sub-bands were divided into 870-880 mhz for non-wireline service and 880-890 for wireline service.
This early allocation of frequencies for cellular communications was found to be inadequate and recently the FCC increased the allocation of frequencies for receive and transmit from twenty megahertz to 25 megahertz. Specifically the receive band was extended to cover 824 mhz to 849 mhz and the transmit band extended to cover 869 mhz to 894 mhz. In order to maintain compatibility with existing equipment, the sub-bands for non-wireline and wireline services had this additional 5 megahertz bandwidth for both receive and transmit split between the non-wireline and wireline services and further, the original sub-band frequencies were not changed. As a result, the non-wireline receive band originally set at 825 to 835 mhz was extended into two receive sub-bands; namely, 824 to 835 mhz and 845 to 846.5 mhz, while the wireline receive sub-band was extended from 835 to 845 mhz to that sub-band plus a sub-band residing between 846.5 and 849 mhz. A similar reallocation of the transmit sub-bands was also made resulting in the non-wireline transmit sub-bands from 869 to 880 mhz and 890 to 891.5 mhz, and wireline transmit sub-bands from the original 880 to 890 mhz and 891.5 to 894 mhz.
As a result of this increase in bandwidth and the resulting addition of two additional sub-bands for reception and transmission, a means for filtering unwanted frequencies for both the non-wireline and wireline services became critical. In particular, with regard to the wireline service, the additional non-wireline 1.5 mhz sub-band which lies between the two wireline sub-bands must be effectively attenuated for wireline reception.
The present invention is a dielectric notch filter which has the desired characteristics of presenting a relatively low impedance having a primarily resistive characteristic within a fairly narrow bandwidth of frequencies while maintaining a relatively small physical size in comparison to other filters. This dielectric notch filter has a high quality factor so as to present little attenuation outside of the desired filtered frequencies.
In particular, the dielectric notch filter described herein uses one or more dielectric notch resonators as set forth in the simultaneously filed co-pending application Ser. No. 284,341 of the present inventors assigned to the same assignee, entitled "DIELECTRIC NOTCH RESONATOR". This application is hereby incorporated by reference.
The dielectric notch filter is achieved by placing these dielectric notch resonators onto a coupling transmission line between the receiver and the antenna so that the dielectric notch resonators are spaced at approximately odd multiples of quarter wavelengths at the frequency of operation. In this manner, interaction between the individual dielectric notch resonators is minimized while each resonator is able to attenuate a band of frequencies about its own center frequency.
The overall result is a dielectric notch filter which can attenuate a desired bandwidth of frequencies such as those described above with regard to cellular communications.